Recombinant terminal deoxynucleotidyl transferase with improved functionality

ABSTRACT

Truncated terminal deoxynucleotidyl transferase (TdT) derivative from calf thymus, characterized in that the derivative in comparison to the native TdT is N-terminally truncated by up to 161 amino acids and has a 20- to 30-fold higher enzyme activity in solutions containing Co 2+  ions, and its recombinant production and use.

[0001] The invention concerns a recombinant N-terminally truncated terminal deoxynucleotidyl transferase (TdT) from calf thymus which, under certain conditions, has an at least 20-fold increased activity compared to the fill-length TdT, as well as the production and use thereof.

[0002] Terminal deoxynucleotidyl transferase (TdT) is a highly conserved enzyme from vertebrates that catalyses the attachment of 5′ triphosphates to the 3′ hydroxyl group of single- or double-stranded DNA. Hence the enzyme acts as a template-independent polymerase (Koiwai et al. (1986), Nucleic Acid Research 14 (14), 5777-5792). In vivo the TdT is responsible for the high diversity of immunoglobulins and T-cell receptors. In addition to the naturally occurring nucleoside triphosphates, TdT usually accepts radioactively labelled triphosphates as well as non-radioactively labelled triphosphates e.g. triphosphates labelled with digoxigenin or biotin. The acceptance of labelled triphosphates also makes the TdT interesting for laboratory and industrial applications (e.g. oligotailing, in situ cell death detection in apoptosis test).

[0003] The full length TdT (molecular weight ca. 58000 Da/520 amino acids) is subject in vivo to a stepwise proteolytic degradation to smaller fragments which are, however, still enzymatically active (Chang et al. (1982), J. Biol. Chem. 257(10): 5700-5706). Starting at the N-terminus, this proteolytic processing generates peptides of 56 kDa, 44 kDa and 42 kDa molecular weight from the 58 kDa TdT which all still have the active centre. Furthermore it is known that the 42 kDa peptide is degraded into an active TdT fragment having a molecular weight of 32 kDa which is in turn composed of 2 peptides, the a peptide of 8 kDa and the β peptide of 26 kDa. This 32 kDa fragment is also the predominant form when isolated from the calf thymus (Chang and Bollum (1986), CRC Crit. Rev. Biochem. 21(1):27-52).

[0004] The isolation of TdT from calf thymus has been known for a long time (Deibel and Coleman (1979) J. Biol. Chem. 254(17): 8634-8640). Moreover the method described by Deibel and Coleman is economical on a large scale since calf thymus is a cheap raw material. However, a disadvantage of this method is that, due to the proteolytic activation mentioned above, it is not possible to purify TdT in a homogeneous, pure-band form nor is it possible to separate the active TdT fragments from inactive TdT fragments. Moreover the isolation of substances from bovine raw materials should nowadays be avoided as far as possible due to the problems associated with BSE.

[0005] The expression of the human full length TdT in E. coli is described in Peterson et al. (1985, J. Biol. Chem. 260(19): 10495-502) and in U.S. Pat. No. 5,037,756 (Bollum et al.). The expressed protein was detected in a crude extract of E. coli using an antibody to TdT (from the rabbit) and was purified as a full length product by immuno-affinity chromatography. However, the yields of TdT in this method are probably rather small; at least no yields and function tests are described.

[0006] In 1988 Chang et al (J. Biol. Chem. 263(25): 12509-12513) describe the expression of full length TdT from humans in a baculovirus system. In this case the yields are about 10% of the total protein content; the enzyme exhibits immunological and enzymatic activity. A disadvantage of this method is the relatively low yields that often occur with heterologous protein expression in the baculovirus system compared to prokaryotic expression systems and the higher production costs of a cell culture fermentation.

[0007] Yang et al. (1995; Nucleic Acids Research 23(11): 2041-2048) describe the expression of the full length TdT from chickens in E. coli. The recombinant TdT was cloned into the vector pET16b and was fused with a His tag to facilitate purification. Although analysis of the recombinant TdT after isolation resulted in a full length product, it had a 2-fold lower activity than native chicken TdT. In the opinion of the authors the reason for the lower activity of the recombinant TdT is either that the His tag interferes with the activity or it is due to the absence of a posttranslational modification.

[0008] Boule et al. describe (1998, Molecular Biotechnology 10: 199-208) the expression of the full length TdT from the mouse in E. coli. The use of a strong promoter (T7 promoter) to increase the expression rate led at first to a high proportion of inactive expressed product (inclusion bodies). The authors subsequently avoided this by drastically decreasing the growth temperature during the induction phase (15° C.). However, a drawback of this is the retarded growth of the E. coli cells and the increased foam formation at these fermentation temperatures.

[0009] The object of the present invention was therefore to provide a TdT in a homogeneous form having an adequate enzymatic activity.

[0010] The object underlying the present invention is achieved by a truncated TdT enzyme from which 161 amino acids are missing at the N-terminus and which has an at least 20-fold increased enzymatic activity compared to the full length TdT from calf thymus. In particular those TdT enzymes have proven to be advantageous according to the invention which are shortened at the N-terminus by 100 to 160 amino acids. According to the invention a TdT which is shortened at the N-terminus by 138 amino acids is especially preferred. Such TdT derivatives have a 20- to 30-fold increased enzyme activity in solutions containing Co²⁺ ions compared to the full length TdT from calf thymus. The truncated TdT according to the invention has a molecular weight (SDS-Page) of ca. 36 to 46 kDa, preferably between 40 and 46 kDa and in particular between 44 and 46 kDa.

[0011] Another subject matter of the invention is a method for the recombinant production of TdT from calf thymus which is characterized by the following steps:

[0012] a) transformation of a host cell with a nucleic acid which codes for an N-terminally truncated TdT fragment and is optionally fused with a nucleic acid which codes for a protein tag that facilitates the subsequent purification,

[0013] b) culture of the host cell to express the recombinant truncated TdT under suitable culture conditions for the respective host cell,

[0014] c) isolation of the recombinant truncated TdT from the host cell and

[0015] d) use and examination of the recombinant truncated TdT in a functional assay.

[0016] A host cell in the sense of the invention means any host cell which is able to actively express large amounts of proteins in the cytoplasm. These are usually prokaryotic cells such as Escherichia coli or Bacillus subtilis, but also yeast or fungal cells such as Pichia pastoris, Pichia methylotropha, Hansenula polymorpha, Saccharomyces cerevisiae, Schizosaccharomaces pombae among others or Aspergillus sp. have proven to be suitable. According to the invention Escherichia coli cells are preferably used.

[0017] In principle all fragments that can be derived from the cDNA of calf thymus TdT are potentially suitable as nucleic acids which code for an N-terminally truncated TdT fragment provided they code for a protein with TdT activity. In particular those nucleic acid sequences have proven to be advantageous according to the invention which code for a truncated TdT enzyme which lacks up to 161 amino acids at the N-terminus compared to the wild type enzyme. The following fragments have proven to be especially advantageous according to the invention: SEQ ID NO.: 7, SEQ ID NO.: 9 or SEQ ID NO.: 11.

[0018] The nucleic acid molecule coding for a truncated TdT protein can be fused with a nucleic acid sequence for the expression which codes for a protein that facilitates the subsequent purification of the expressed TdT. Suitable purification protein tags and the DNA fragments that encode them are in principle familiar to a person skilled in the art. In addition to the (poly)His tag, it is for example possible to use biotinylation proteins, streptavidin-binding proteins such as Streptacin® (“Inst. Für Bioanalytik, IBA”, Göttingen/Germany), maltose-binding proteins (e.g. U.S. Pat. No. 5,643,758), GST and HA tags (e.g. U.S. Pat. No. 5,654,176; WO 98/17691) according to the invention.

[0019] Another subject matter of the invention is the purification of the truncated terminal transferase derivatives from the cytoplasm of the host cell. E. coli K12 UT5600 cells which overexpress the terminal transferase gene were used in particular as the starting material for the purification of the recombinant terminal transferase.

[0020] The TdT is usually purified at 4° C. After cell lysis and separation of the nucleic acids, which in principle can be carried out by known methods, a series of chromatographic steps are carried out. According to the invention the fraction freed from nucleic acids is firstly subjected to an ion-exchanger chromatography (cation exchanger) e.g. using a Poros HS 50 column. If a protein tag serving as a purification aid such as a (poly)His, a biotinylation peptide, Streptacin® or a maltose-binding protein is linked to the TdT derivative, i.e. has been co-expressed, an affinity chromatography is subsequently carried out. In the case of a TdT expression product linked to a (poly)His peptide, the commercially available nickel-chelate columns are especially suitable for this purification step. The resulting TdT fraction is subsequently further purified by a suitable hydrophobic chromatography for example on phenyl Sepharose fast flow (ff). The described purification method yields a very pure terminal transferase which is free from contaminating enzyme activities. The purity of Δ138-TdT having a molar mass of 45.3 kDa in an SDS gel is shown as an example in FIG. 1.

[0021] The truncated recombinant terminal transferase peptides according to the invention surprisingly have a substantially higher enzymatic activity in the activity test in solutions containing cobalt (Co²⁺) ions (the so-called Co system) than native terminal transferase derivatives. The native terminal transferase exhibits a ca. three- to four-fold higher enzymatic activity in the Co system compared to the Zn/Mg system. In contrast the recombinant N-terminally truncated TdT derivatives exhibit a 20- to 30-fold increased activity. For example the Δ138-TdT derivative has a ca. 23-fold higher activity in the Co system than in the Zn/Mg system. Hence in contrast to native terminal transferase, the recombinant Δ138-TdT has a significantly higher enzyme activity in the Co system.

[0022] Moreover it is surprising that the recombinant Δ138-TdT derivative has a significantly better performance in the function test than the native terminal transferase. A 30 mer oligonucleotide (5′-pTTG GGT AAC GCC AGG GTT TTC CCA GTC ACG OH-3′) was used as a template for the tailing reaction. After the reaction was completed, the reaction products of the tailing experiment were separated on a 6% agarose gel and evaluated (FIG. 2). The recombinant TdT resulted in a longer and hence better product of the tailing reaction.

FIGURE LEGENDS

[0023]FIG. 1: SDS gel electrophoresis of the purified terminal transferase (lane 1: molecular weight marker 12 (Novex Co.); lane 2: terminal transferase, 10 units (Zn/Mg system)

[0024]FIG. 2: Oligo tailing reaction (lanes 1, 4: oligonucleotide; lane 2: product of the tailing reaction, 10 units TdT, native; lane 3: product of the tailing reaction, 10 units TdT, recombinant; LSV: DNA molecular weight marker V (pBR 322 DNA cleaved with Hae III, 22 fragments 8-587 bp; Roche Diagnostics GmbH, Cat. No. 821 705))

[0025] Legends for the sequence protocols:

[0026] SEQ ID NO.: 1 cDNA sequence of the TdT from calf thymus (pos. 22-1581)

[0027] SEQ ID NO.: 2 amino acid sequence of the TdT from calf thymus (520 AA)

[0028] SEQ ID NO.: 3 5′ primer (58 N)

[0029] SEQ ID NO.: 4 5′ primer (63 N)

[0030] SEQ ID NO.: 5 5′ primer (60 N)

[0031] SEQ ID NO.: 6 3′ primer (42 N)

[0032] SEQ ID NO.: 7 nucleic acid sequence coding for the truncated TdT, Δ138-TdT with His tag (1187 N)

[0033] SEQ ID NO.: 8 amino acid sequence of the truncated TdT, Δ138-TdT with His tag (392 AA)

[0034] SEQ ID NO.: 9 nucleic acid sequence coding for the truncated TdT, Δ152-TdT with His tag (1148 N)

[0035] SEQ ID NO.: 10 amino acid sequence of the truncated TdT, Δ152-TdT with His tag (379 AA)

[0036] SEQ ID NO.: 11 nucleic acid sequence coding for the truncated TdT, Δ161-TdT with His tag (1121 N)

[0037] SEQ ID NO.: 12 amino acid sequence of the truncated TdT, Δ161-TdT with His tag (370 AA).

[0038] The invention is further elucidated by the following examples.

[0039] Recombinant DNA Technique

[0040] Standard methods were used to manipulate DNA as described by Sambrook, J. et al. (1989) Molecular cloning: A laboratory manual. Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y. The recommendations of the manufacturer were followed when using kits. The molecular biological reagents were used according to the instructions of the manufacturer.

EXAMPLE 1

[0041] Generation of Truncated TdT Genes:

[0042] Oligonucleotides according to SEQ ID 3-6 which enable the isolation of truncated genes from the complete reading frame by means of PCR, were designed on the basis of a cDNA clone (Kowai et al. (1986), Nucleic Acids Res. 14: 5777-5792) having an insert of 1923 bp according to SEQ ID NO.: 1 which contains the complete reading frame of the gene that codes for the terminal transferase according to SEQ ID NO.: 2 from calf thymus. Each of the 5′ primers (SEQ ID NO.: 3-5).were designed such that they contain the codons for the amino acids Met-Arg-Gly-Ser-His-His-His-His-His-His downstream of the coding region and hence the truncated TdT peptides are N-terminally fused with a His tag. The 5′ primers contain no recognition sequence for a restriction endonuclease, but should be cloned at the 5′ end via a blunt end whereas the 3′ primer has the recognition sequence for the restriction endonuclease HindIII downstream of the coding region.

[0043] In order to isolate a gene which codes for a TdT truncated by 138 amino acids at the N-terminus (Δ138-TdT), the PCR reaction was carried out using the oligonucleotides according to SEQ ID NO.: 3 (5′ primer) and SEQ ID NO.: 6 (3′ primer). The resulting PCR product according to SEQ ID NO.: 7 which codes for the Δ138-TdT with a His tag according to SEQ ID NO.: 8 was examined by sequencing.

[0044] In order to isolate a gene which codes for a TdT truncated by 152 amino acids at the N-terminus (Δ152-TdT), the PCR reaction was carried out using the oligonucleotides according to SEQ ID NO.: 4 (5′ primer) and SEQ ID NO.: 6 (3′ primer).

[0045] The resulting PCR product according to SEQ ID NO.: 9 which codes for the Δ152-TdT with a His tag according to SEQ ID NO.: 10 was examined by sequencing.

[0046] In order to isolate a gene which codes for a TdT truncated by 161 amino acids at the N-terminus (Δ161-TdT), the PCR reaction was carried out using the oligonucleotides according to SEQ ID NO.: 5 (5′ primer) and SEQ ID NO.: 6 (3′ primer). The resulting PCR product according to SEQ ID NO.: 11 which codes for the Δ161-TdT with a His tag according to SEQ ID NO.: 12 was examined by sequencing.

[0047] Construction of the Expression Plasmids for the Truncated TdT Peptides

[0048] In order to express the TdT, the truncated genes were each cloned into expression vectors in such a manner that the structural genes were each inserted in the correct orientation under the control of a suitable promoter, preferably an IPTG-inducible promoter such as lac, lacUV5, tac or T5 promoter, particularly preferably the lac promoter. For this purpose the respective PCR product was recleaved at the 3′ end with HindIII whereas the 5′ end was not changed, the restriction mixtures were separated by agarose gel electrophoresis and the 1181 bp fragment was isolated from the agarose gel for the Δ138 TdT gene, the 1139 bp fragment was isolated for the Δ152 TdT gene and the 1115 bp fragment was isolated for the Δ161 TdT gene, each of which contained a nucleic acid which codes for the His tag. Various expression plasmids such as pUC, pDS, pQE, pKK but preferably pUC18 (Yanisch-Perron et al., (1985) Gene 33: 103-119) were used for the expression. In order to insert the genes for the truncated TdT peptides pUC18 was firstly cleaved with EcoRI (Roche Diagnostics) according to the manufacturer's instructions, the restriction endonuclease EcoRI was inactivated by incubating at 65° C. for 15 min and the resulting overhanging ends were filled in to form a blunt end using Klenow polymerase (Roche Diagnostics) according to the manufacturer's instructions. The Klenow polymerase was inactivated by incubating again at 65° C. for 15 min. Subsequently the vector fragment was cleaved with HindIII (Roche Diagnostics), the restriction mixture was separated by agarose gel electrophoresis and the resulting vector fragment of ca. 2656 bp was isolated from the agarose gel. The vector fragment obtained in this manner was separated and ligated with the isolated PCR products for the truncated TdT peptides. The correct insertion of the genes was checked by means of restriction control and sequencing. The resulting plasmids pUC18Δ138-TdT, pUC18Δ152-TdT and pUC18Δ161-TdT (see FIG. 1) were cotransformed separately in various E. coli strains together with the helper plasmid pUBS520. The helper plasmid pUBS520 (Brinkmann et al., 1989, Gene 85: 109-114) carries among others the lacI^(q) gene which codes for the lac repressor and the dnaY gene which codes for the rare tRNA^(ARG) in E. coli (recognizes the codons AGA and AGG) (Garcia et al., 1986, Cell 45: 453-459). The kanamycin resistance gene from the transposon TN903 is used as the selection marker.

EXAMPLE 2

[0049] Transformation of the Expression Plasmids pUC18Δ`38-TdT, pUC18Δ152-TdT and pUC18Δ161-TdT into Various E. coli Expression Strains

[0050] Competent cells of various E. coli strains were produced according to the method of Hanahan (J. Mol. Biol. 1993, vol. 166: 557). 200 μl of cells prepared in this manner were admixed with 20 ng isolated expression plasmid DNA pUC18Δ138-TdT, pUC18Δ152-TdT and pUC18Δ161-TdT and 40 ng helper plasmid DNA. After 30 min incubation on ice, they were subjected to a heat shock (90 sec at 42° C.). Subsequently the cells were transferred to 1 ml LB medium and incubated for 1 hour at 37° C. in the LB medium for the phenotypic expression. Aliquots of this transformation mixture were plated out on LB plates containing ampicillin and kanamycin as selection markers and incubated for 15 hours at 37° C. Preferred strains are E. coli K12 C600, DH5α, LE392, JM83, JM105, NM522, M15, RR1Δ15, UT5600, TG1, A1200 or the strains E. coli B, BL21, HB101, Escherichia coli UT5600 is particularly preferred.

EXAMPLE 3

[0051] Expression of the Truncated TdT Genes in E. coli

[0052] In order to express the gene which codes for the truncated TdT peptides, plasmid-containing clones were inoculated in 3 ml LB_(ampkan) medium and incubated at 37° C. in a shaker. The cells were induced with 0.5 mM IPTG at an optical density of 0.5 (measured at 550 nm, OD₅₅₀) and incubated for 4 h at 37° C. in a shaker. Subsequently the optical density of the individual expression clones was determined, an aliquot corresponding to an OD_(550 nm) of 5.0/ml was removed and the cells were centrifuged (10 min, 6000 rpm, 4° C.). The cell pellet was resuspended in 400 μl TE buffer (50 mM Tris/50 mM EDTA, pH 8.0), the cells were disrupted by ultrasound and the soluble protein fraction was separated from the insoluble protein fraction by centrifugation (10 min, 13800 rpm, 4° C.). Application buffer containing SDS and 2-mercapto-ethanol was added to all fractions and the proteins were denatured by boiling (5 min at 100° C.). Subsequently 10 μl aliquots were analysed by means of an analytical SDS gel (10%) (Laemmli U.K. 1970 Nature 227: 555-557).

[0053] The evaluation of the SDS gel showed that there is a clear overexpression of the truncated TdT fragments. An overexpressed additional band is observed at ca. 45 kDa (Δ138-TdT) or 44 kDa (Δ152-TdT) or 43 kDa (Δ161-TdT) which does not appear in the non-induced or induced but non-plasmid-containing control clones. All TdT fragments were detected in the soluble protein fraction when using this expression strategy even at high growth temperatures whereas corresponding bands at the same level were not detected in the insoluble protein fraction.

EXAMPLE 4

[0054] Determination of Terminal Transferase Activity

[0055] Various tests were carried out to determine terminal transferase activity.

[0056] 1. Non-Radioactive Test (Test A)

[0057] The terminal transferase activity was detected in the fractions during purification by means of a non-radioactive test system. The DIG Oligo 3′ end labeling Kit (Roche Diagnostics GmbH, Cat. No. 1 362 372) was used for this. The incubation time was extended to 30 minutes.

[0058] 2. Radioactive Test Systems

[0059] A. Test in the Zn/Mg System (Test B)

[0060] The terminal transferase activity of the pools was determined by a radioactive test system which contained zinc and magnesium ions. The test for terminal transferase activity was carried out in a test volume of 60 μl (40 mM potassium cacodylate, pH 6.8, 0.33 mM ZnSO₄, 10 mM MgCl₂, 1 mM dATP, 0.1 AB poly d(pT)₆, 12.5 pM [3H]-dATP). Terminal transferase (10 μl) was added at suitable dilutions. After incubating for 30 min at 37° C., the reaction was stopped with 10% TCA solution (1000 μl). The radioactively-labelled product that formed was washed after precipitation on nitrocellulose filters. The rate of incorporation of radioactivity was measured in a scintilation counter and the terminal transferase activity of the sample was calculated. In this connection one enzyme unit was defined as the amount of terminal transferase which results in the incorporation of 1.0 nMol DAMP into acid-insoluble product within 60 min at 37° C.

[0061] This test is used to routinely determine the activity of native and recombinant terminal transferase.

[0062] B. Test in the Co System (Test C)

[0063] The terminal transferase activity was also determined using a test system which contained cobalt ions. This test was carried out in a test volume of 120 μl (200 mM potassium cacodylate, pH 7.2, 1 mM CoCl₂, 1 mM dTTP, 0.1 AB poly d(pT)₆, 37.5 pMol [3H]-dTTP). Terminal transferase (10 μl) was added at suitable dilutions. After incubating for 30 min at 37° C., the reaction was stopped with 10% TCA solution (1000 μl). The radioactively-labelled product that formed was washed after precipitation on nitrocellulose filters. The rate of incorporation of radioactivity was measured in a scintilation counter and the terminal transferase activity of the samples was calculated. In this connection one enzyme unit was defined as the amount of terminal transferase which results in the incorporation of 1.0 nMol dTMP or dATP into acid-insoluble product within 60 min at 37° C. using d(pT)₆ as a primer.

[0064] Test for Contaminating Activities

[0065] The test for the presence of contaminating foreign activities was carried out in a solution consisting of 10 mM Tris/HCl, pH 7.5, 10 mM MgCl₂, 1 mM DTE.

[0066] Suitable samples of the individual enzyme fractions were incubated with the corresponding nucleic acids. So-called nicking activity was detected by incubation with the plasmid pBR322 (1 μg) for 2-16 hours at 37° C. Unspecific nucleases were detected by incubation with lambda DNA/EcoRI, HindIII (1 μg) for 2-16 hours at 37° C.

[0067] For the test for contamination with exonucleases the samples were incubated for 4 hours at 37° C. with 4 μg [3H]-labelled DNA and afterwards the released [3H] -labelled nucleotides were determined.

EXAMPLE 5

[0068] Purification of Terminal Transferase

[0069]E. coli K12 UT5600 cells which overexpressed the terminal transferase gene (see above) were used as the starting material for purifying recombinant terminal transferase.

[0070] TdT was purified at 4° C. The purification was carried out after cell lysis and separation of the nucleic acids by a series of chromatographic steps. The purification process yields a recombinant TdT which is free from contaminating enzyme activities.

[0071] Solutions Used:

[0072] buffer A: 50 mM Tris/HCl, pH 7.6, 0.5 M NaCl, 50 mM LiCl

[0073] buffer B: 50 mM KPO₄, pH 6.0, 5% glycerol

[0074] buffer C: 50 mM Tris/HCl, pH 7.6, 0.5 M NaCl, 5% glycerol,

[0075] buffer D: 20 mM KPO₄, pH 7.0, 1.3 M ammonium sulfate, 5% glycerol storage buffer: 60 mM KPO₄, pH 7.2,150 mM KCl, 1 mM 2-mercaptoethanol, 0.5 % Triton X-100, 50% glycerol.

[0076] Cell Lysis:

[0077] Ca. 1100 g cells of E. coli K12 UT5600 were admixed with 4000 ml buffer A, thawed and suspended. 20 ml 0.1 M PMSF solution (Roche Diagnostics GmbH, Cat. No. 236 608) was added to the suspension. The cells were subsequently lysed by means of high pressure dispersion (Gaulin Lab-60) while cooling (temperature: <10° C.). This resulted in a typical degree of lysis of the cell suspension of 40-50%.

[0078] Precipitation of Nucleic Acids:

[0079] The nucleic acids were subsequently removed by means of Polymin precipitation. 100 ml of a 10% Polymin-P solution was added dropwise. In the case of an incomplete precipitation, an additional dropwise addition was carried out. The centrifugation (30 min, 5000 rpm, 4° C.) was carried out after incubation for 30 min at 4° C.

[0080] Chromatographic Purifications:

[0081] Chromatography on Poros HS 50 Column:

[0082] The dialysed centrifugation supernatant was applied to a Poros HS 50 column (9 cm×20 cm, PerSeptiv) equilibrated with buffer B+0.2 M NaCl and washed with ca. 101 buffer B+0.2 M NaCl. The enzyme was eluted with a linear gradient of buffer B+200 mM NaCl and buffer B+1 NaCl in a total volume of 81. The flow rate was 100 ml per min, the fraction size was 100 ml. The terminal transferase elutes at an NaCl concentration of 300 mM to 700 mM.

[0083] Affinity Chromatography on Ni-Chelate Column:

[0084] The dear pool was adjusted to pH 7.5 with K₂HPO₄ and admixed with 1/100 buffer C+1 M imidazole and subsequently applied to a chelating Sepharose ff column (2.6 cm×10 cm, Pharmacia) equilibrated with buffer C+10 mM imidazole and loaded with nickel; it was afterwards washed with ca. 800 ml buffer C+20 mM imidazole, then washed with buffer C+30 mM imidazole. The enzyme was eluted with a linear gradient of buffer C+30 ml imidazole and buffer C+1 M imidazole in a total volume of 600 ml. The flow rate was 12 ml per minute and the fraction size was 25 ml per fraction. The enzyme eluted at a concentration of 50 mM to 200 mM imidazole.

[0085] All active fractions were pooled. Solid ammonium sulfate was added to the pool to a final concentration of 1.3 M.

[0086] Chromatography on Phenyl Sepharose ff:

[0087] The pool was then applied to a phenyl Sepharose ff column (2.6 cm×10 cm, Pharmacia) equilibrated with buffer D. The column was firstly washed with ca. 400 ml buffer D and then with ca. 600 ml buffer D+500 mM ammonium sulfate. The enzyme was eluted in this washing step. The flow rate was 10 ml per min, the fraction size was 10 ml.

[0088] The active fractions were pooled and dialysed against storage buffer. In order to analyse the purity, application buffer containing SDS and 2-mercaptoethanol was added to the purified protein and the sample was denatured by boiling (5 min 100° C.). Subsequently a sample (20 μl) was analysed by means of an analytical SDS gel (4-20%) (Laemmli U.K. 1970 Nature 227: 555-557). The described purification method yields a highly pure terminal transferase having a molar mass of 45.3 kDa (FIG. 1).

EXAMPLE 6

[0089] Comparison of the Activities of Native and Recombinant Terminal Transferases

[0090] Due to the improved performance of the recombinant terminal transferase in the tailing reaction, the enzyme activities of the two terminal transferases were examined in two different test systems. The Zn/Mg system (test B) and the Co system (test C) were used for this. TABLE 1 Activities of native and recombinant terminal transferase in different test systems (Zn/Mg system and Co system) 1st Test 2nd Test 3rd Test 4th Test mean (U/μl) (U/μl) (U/μl) (U/μl) (U/μl) a) Zn/Mg-system TdT, recombinant 36.0 25.8 31.3 27.9 30.3 TdT, native 99.0 85.6 86.9 92.3 91 b) Co system TdT recombinant 841 668 676 705 722.5 TdT, native 452 436 273 312 368.3

[0091] The native terminal transferase has a ca. three to four-fold higher activity in the Co system than in the Zn/Mg system. In contrast the recombinant terminal transferase has a ca. 23-fold higher activity in the Co system than in the Zn/Mg system. In comparison to the native terminal transferase the recombinant terminal transferase thus has a more pronounced improvement of the enzyme activity in the Co system.

[0092] This difference in activity could be the explanation for the improved performance of the recombinant terminal transferase in the tailing reaction.

EXAMPLE 7

[0093] Function Test for Terminal Transferase

[0094] The recombinant terminal transferase that was obtained was examined in a function test. The function test consists of an oligo tailing reaction. For this 10 units of the recombinant TdT and of the native TdT was used in a “Dig Oligonucleotide Tailing” kit (Cat. No. 1 417 231, Roche Diagnostics GmbH). 100 pmol of a 30 mer oligonucleotide (5′-pTTG GGT AAC GCC AGG GTT TTC CCA GTC ACG OH-3′) was used as the template for the tailing reaction.

[0095] The reaction products of the tailing experiment were separated on a 6% agarose gel and evaluated (FIG. 2). The recombinant TdT resulted in a longer and hence better product of the tailing reaction.

[0096] Comparison of TdT According to the Invention with Known Preparations TABLE 2 Activities of native and recombinant TdT and of TdT preparations of various manufacturers in different test systems. Zn/Mg system Co system preparation/lot source [U/μl] [U/μl] truncated TdT E. coli, rec. 30, 3 722, 5  (calf thymus) TdT, native calf thymus 91, 0 368, 3  [55 U/μl] TdT, native calf thymus  9, 4 47, 8 Stratagene Lot: 0610233 [28 U/μl] TdT, native calf thymus 11, 3 72, 3 Amersham Pharmacia Lot: 5473 [15 U/μl] TdT Baculovirus, rec.  2, 7 20, 4 BRL (calf thymus) Lot: 1093333 ]15 U/μl] TdT, native calf thymus 39, 9 70, 9 Promega Lot: 91884 [20 U/μl] TdT E. coli, rec. 24, 7 42, 4 NEBL (calf thymus) Lot: 2A [20 U/μl]

[0097] Literature:

[0098] Brinkmann U., Mattes R. E. und Buckel P. (1989), Gene 85: pp. 109-114

[0099] Boulé J. -B., Johnson E., Rougeon F. und Papanicolaou C. (1998), Molecular Biotechnology 10: pp. 199-208

[0100] Chang L. M., Plevani P. und Bollum F. J. (1982), J. Biol. Chem. 257(10): pp. 5700-5706

[0101] Chang L. M. und Bollum F. J. (1986), CRC Crit Rev Biochem 21(1): pp. 27-52

[0102] Chang L. M., Rafter E., Rusquet-Valerius, Peterson R. C., White S. T. und Bollum F. J. (1986), J. Biol. Chem. 263 (25): pp. 12509-12513

[0103] Deibel Jr. M. R. und Coleman M. S.(1979), J. Biol. Chem. 254(17): pp 8634-8640

[0104] Garcia G. M., Mar P. K., Mullin D. A., Walker J. R. und Prather N. E (1986), Cell 45: pp.453-459

[0105] Hanahan D. (1983), J. Mol. Biol. Vol. 166 pp. 557

[0106] Koiwai O., Yokota T., Kageyama T., Hirose T., Yoshida S. und Arai K. -I. (1986), Nucleic Acid Research 14 (14), pp. 5777-5792

[0107] Laemmli U. K. (1970), Nature 227: pp. 555-557

[0108] Peterson R. C., Cheung L. C., Mattaliano R. J., White S. T., Chang L. M. S. und Bollum F. J. (1985), J Biol Chem 260 (19):pp10495-502

[0109] Sambrook J., Fritsch E. F. und Maniatis T., (1989), In Molecular cloning: A Laboratory Manual second Edition Cold Spring Harbor Laboratory Press N.Y. (USA)

[0110] Yang B., Gathy K. N. und Coleman M. S. (1995); Nucleic Acids Research 23 (11): pp. 2041-2048

[0111] U.S. Pat. No. 5,037,756 Inventors Bollum F. J., Chang L. M. S. und Peterson R. C.

[0112]

1 12 1 1923 DNA Calf thymus 1 ctcttctgga gataccactt gatggcacag cagaggcagc atcagcgtct tcccatggat 60 ccgctgtgca cagcctcctc aggccctcgg aagaagagac ccaggcaggt gggtgcctca 120 atggcctccc ctcctcatga catcaagttt caaaatttgg tcctcttcat tttggagaag 180 aaaatgggaa ccacccgcag aaacttcctc atggagctgg ctcgaaggaa aggtttcagg 240 gttgaaaatg agctcagtga ttctgtcacc cacattgtag cagaaaacaa ctctggttca 300 gaggttctcg agtggcttca ggtacagaac ataagagcca gctcgcagct agaactcctt 360 gatgtctcct ggctgatcga aagtatggga gcaggaaaac cagtggagat tacaggaaaa 420 caccagcttg ttgtgagaac agactattca gctaccccaa acccaggctt ccagaagact 480 ccaccacttg ctgtaaaaaa gatctcccag tacgcgtgtc aaagaaaaac cactttgaac 540 aactataacc acatattcac ggatgccttt gagatactgg ctgaaaattc tgagtttaaa 600 gaaaatgaag tctcttatgt gacatttatg agagcagctt ctgtacttaa atctctgcca 660 ttcacaatca tcagtatgaa ggatacagaa ggaattccct gcctggggga caaggtgaag 720 tgtatcatag aggaaattat tgaagatgga gaaagttctg aagttaaagc tgtgttaaat 780 gatgaacgat atcagtcctt caaactcttt acttctgttt ttggagtggg actgaagaca 840 tctgagaaat ggttcaggat ggggttcaga tctctgagta aaataatgtc agacaaaacc 900 ctgaaattca caaaaatgca gaaagcagga tttctctatt atgaagacct tgtcagctgc 960 gtgaccaggg ccgaagcaga ggcggttggc gtgctggtta aagaggctgt gtgggcattt 1020 ctgccggatg cctttgtcac catgacagga ggattccgca ggggtaagaa gattgggcat 1080 gatgtagatt ttttaattac cagcccagga tcagcagagg atgaagagca acttttgcct 1140 aaagtgataa acttatggga aaaaaaggga ttacttttat attatgacct tgtggagtca 1200 acatttgaaa agttcaagtt gccaagcagg caggtggata ctttagatca ttttcaaaaa 1260 tgctttctga ttttaaaatt gcaccatcag agagtagaca gtagcaagtc caaccagcag 1320 gaaggaaaga cctggaaggc catccgtgtg gacctggtta tgtgccccta cgagaaccgt 1380 gcctttgccc tgctaggctg gactggctcc cggcagtttg agagagacat ccggcgctat 1440 gccacacacg agcggaagat gatgctggat aaccacgctt tatatgacaa gaccaagagg 1500 gtatttctca aagcggaaag tgaagaagaa atctttgcac atctgggatt ggactacatt 1560 gaaccatggg aaagaaatgc ttaggagaaa gctgtcaact tttttctttt ctgttctttt 1620 tttcaggtta gacaaattat gcttcatatt ataatgaaag atgccttagt caagtttggg 1680 attctttaca ttttaccaag atgtagattg cttctagaaa taagtagttt tggaaacgtg 1740 atcaggcacc ccctgggtta tgctctggca agccatttgc aggactgatg tgtagaactc 1800 gcaatgcatt ttccatagaa acagtgttgg aattggtggc tcatttccag ggaagttcat 1860 caaagcccac tttgcccaca gtgtagctga aatactgtat acttgccaat aaaaatagga 1920 aac 1923 2 520 PRT Calf thymus 2 Met Ala Gln Gln Arg Gln His Gln Arg Leu Pro Met Asp Pro Leu Cys 1 5 10 15 Thr Ala Ser Ser Gly Pro Arg Lys Lys Arg Pro Arg Gln Val Gly Ala 20 25 30 Ser Met Ala Ser Pro Pro His Asp Ile Lys Phe Gln Asn Leu Val Leu 35 40 45 Phe Ile Leu Glu Lys Lys Met Gly Thr Thr Arg Arg Asn Phe Leu Met 50 55 60 Glu Leu Ala Arg Arg Lys Gly Phe Arg Val Glu Asn Glu Leu Ser Asp 65 70 75 80 Ser Val Thr His Ile Val Ala Glu Asn Asn Ser Gly Ser Glu Val Leu 85 90 95 Glu Trp Leu Gln Val Gln Asn Ile Arg Ala Ser Ser Gln Leu Glu Leu 100 105 110 Leu Asp Val Ser Trp Leu Ile Glu Ser Met Gly Ala Gly Lys Pro Val 115 120 125 Glu Ile Thr Gly Lys His Gln Leu Val Val Arg Thr Asp Tyr Ser Ala 130 135 140 Thr Pro Asn Pro Gly Phe Gln Lys Thr Pro Pro Leu Ala Val Lys Lys 145 150 155 160 Ile Ser Gln Tyr Ala Cys Gln Arg Lys Thr Thr Leu Asn Asn Tyr Asn 165 170 175 His Ile Asp Ala Phe Glu Ile Leu Ala Glu Asn Ser Glu Phe Lys Glu 180 185 190 Asn Glu Val Ser Tyr Val Thr Phe Met Arg Ala Ala Ser Val Leu Lys 195 200 205 Ser Leu Pro Phe Thr Ile Ile Ser Met Lys Asp Thr Phe Thr Glu Gly 210 215 220 Ile Pro Cys Leu Gly Asp Lys Val Lys Cys Ile Ile Glu Glu Ile Ile 225 230 235 240 Glu Asp Gly Glu Ser Ser Glu Val Lys Ala Val Leu Asn Asp Glu Arg 245 250 255 Tyr Gln Ser Phe Lys Leu Ser Val Phe Gly Val Gly Leu Lys Thr Ser 260 265 270 Glu Lys Trp Phe Arg Met Gly Phe Thr Phe Arg Ser Leu Ser Lys Ile 275 280 285 Met Ser Asp Lys Thr Leu Lys Lys Met Gln Lys Ala Gly Phe Leu Tyr 290 295 300 Tyr Glu Asp Leu Val Ser Cys Val Thr Arg Ala Glu Ala Glu Ala Val 305 310 315 320 Gly Val Leu Val Lys Glu Ala Val Trp Ala Phe Leu Pro Asp Ala Phe 325 330 335 Val Thr Met Thr Gly Gly Phe Arg Arg Gly Lys Lys Ile Gly His Asp 340 345 350 Val Asp Phe Leu Ile Thr Ser Pro Gly Ser Ala Glu Asp Glu Glu Gln 355 360 365 Leu Leu Pro Lys Val Ile Asn Leu Trp Glu Lys Lys Gly Leu Leu Leu 370 375 380 Tyr Tyr Asp Leu Val Glu Ser Thr Phe Glu Lys Phe Lys Phe Thr Leu 385 390 395 400 Pro Ser Arg Gln Val Asp Thr Leu Asp His Phe Gln Lys Cys Phe Leu 405 410 415 Ile Leu Lys Leu His His Gln Arg Val Asp Ser Ser Lys Ser Asn Gln 420 425 430 Gln Glu Gly Lys Thr Trp Lys Ala Ile Arg Val Asp Leu Val Met Cys 435 440 445 Pro Tyr Glu Asn Arg Ala Phe Ala Leu Leu Gly Trp Thr Gly Ser Arg 450 455 460 Gln Phe Glu Arg Asp Ile Arg Arg Tyr Ala Thr His Glu Arg Lys Met 465 470 475 480 Met Leu Asp Asn His Ala Leu Tyr Asp Lys Thr Lys Arg Val Phe Leu 485 490 495 Lys Ala Glu Ser Glu Glu Glu Ile Phe Ala His Leu Gly Leu Asp Tyr 500 505 510 Ile Glu Pro Trp Glu Arg Asn Ala 515 520 3 58 DNA Artificial Sequence Primer sequence 3 atgagaggat cgcatcacca tcaccatcac agaacagact attcagctac cccaaacc 58 4 63 DNA Artificial Sequence Primer sequence 4 atgagaggat cgcatcacca tcaccatcac aagactccac cacttgctgt aaaaaagatc 60 tcc 63 5 60 DNA Artificial Sequence Primer sequence 5 atgagaggat cgcatcacca tcaccatcac atctcccagt acgcgtgtca aagaaaaacc 60 6 42 DNA Artificial Sequence Primer sequence 6 gcgcaagctt aagcatttct ttcccatggt tcaatgtagt cc 42 7 1187 DNA Calf thymus 7 atgagaggat cgcatcacca tcaccatcac agaacagact attcagctac cccaaaccca 60 ggcttccaga agactccacc acttgctgta aaaaagatct cccagtacgc gtgtcaaaga 120 aaaaccactt tgaacaacta taaccacata ttcacggatg cctttgagat actggctgaa 180 aattctgagt ttaaagaaaa tgaagtctct tatgtgacat ttatgagagc agcttctgta 240 cttaaatctc tgccattcac aatcatcagt atgaaggata cagaaggaat tccctgcctg 300 ggggacaagg tgaagtgtat catagaggaa attattgaag atggagaaag ttctgaagtt 360 aaagctgtgt taaatgatga acgatatcag tccttcaaac tctttacttc tgtttttgga 420 gtgggactga agacatctga gaaatggttc aggatggggt tcagatctct gagtaaaata 480 atgtcagaca aaaccctgaa attcacaaaa atgcagaaag caggatttct ctattatgaa 540 gaccttgtca gctgcgtgac cagggccgaa gcagaggcgg ttggcgtgct ggttaaagag 600 gctgtgtggg catttctgcc ggatgccttt gtcaccatga caggaggatt ccgcaggggt 660 aagaagattg ggcatgatgt agatttttta attaccagcc caggatcagc agaggatgaa 720 gagcaacttt tgcctaaagt gataaactta tgggaaaaaa agggattact tttatattat 780 gaccttgtgg agtcaacatt tgaaaagttc aagttgccaa gcaggcaggt ggatacttta 840 gatcattttc aaaaatgctt tctgatttta aaattgcacc atcagagagt agacagtagc 900 aagtccaacc agcaggaagg aaagacctgg aaggccatcc gtgtggacct ggttatgtgc 960 ccctacgaga accgtgcctt tgccctgcta ggctggactg gctcccggca gtttgagaga 1020 gacatccggc gctatgccac acacgagcgg aagatgatgc tggataacca cgctttatat 1080 gacaagacca agagggtatt tctcaaagcg gaaagtgaag aagaaatctt tgcacatctg 1140 ggattggact acattgaacc atgggaaaga aatgcttaag cttgcgc 1187 8 392 PRT Calf thymus 8 Met Arg Gly Ser His His His His His His Arg Thr Asp Tyr Ser Ala 1 5 10 15 Thr Pro Asn Pro Gly Phe Gln Lys Thr Pro Pro Leu Ala Val Lys Lys 20 25 30 Ile Ser Gln Tyr Ala Cys Gln Arg Lys Thr Thr Leu Asn Asn Tyr Asn 35 40 45 His Ile Asp Ala Phe Glu Ile Leu Ala Glu Asn Ser Glu Phe Lys Glu 50 55 60 Asn Glu Val Ser Tyr Val Thr Phe Met Arg Ala Ala Ser Val Leu Lys 65 70 75 80 Ser Leu Pro Phe Thr Ile Ile Ser Met Lys Asp Thr Phe Thr Glu Gly 85 90 95 Ile Pro Cys Leu Gly Asp Lys Val Lys Cys Ile Ile Glu Glu Ile Ile 100 105 110 Glu Asp Gly Glu Ser Ser Glu Val Lys Ala Val Leu Asn Asp Glu Arg 115 120 125 Tyr Gln Ser Phe Lys Leu Ser Val Phe Gly Val Gly Leu Lys Thr Ser 130 135 140 Glu Lys Trp Phe Arg Met Gly Phe Thr Phe Arg Ser Leu Ser Lys Ile 145 150 155 160 Met Ser Asp Lys Thr Leu Lys Lys Met Gln Lys Ala Gly Phe Leu Tyr 165 170 175 Tyr Glu Asp Leu Val Ser Cys Val Thr Arg Ala Glu Ala Glu Ala Val 180 185 190 Gly Val Leu Val Lys Glu Ala Val Trp Ala Phe Leu Pro Asp Ala Phe 195 200 205 Val Thr Met Thr Gly Gly Phe Arg Arg Gly Lys Lys Ile Gly His Asp 210 215 220 Val Asp Phe Leu Ile Thr Ser Pro Gly Ser Ala Glu Asp Glu Glu Gln 225 230 235 240 Leu Leu Pro Lys Val Ile Asn Leu Trp Glu Lys Lys Gly Leu Leu Leu 245 250 255 Tyr Tyr Asp Leu Val Glu Ser Thr Phe Glu Lys Phe Lys Phe Thr Leu 260 265 270 Pro Ser Arg Gln Val Asp Thr Leu Asp His Phe Gln Lys Cys Phe Leu 275 280 285 Ile Leu Lys Leu His His Gln Arg Val Asp Ser Ser Lys Ser Asn Gln 290 295 300 Gln Glu Gly Lys Thr Trp Lys Ala Ile Arg Val Asp Leu Val Met Cys 305 310 315 320 Pro Tyr Glu Asn Arg Ala Phe Ala Leu Leu Gly Trp Thr Gly Ser Arg 325 330 335 Gln Phe Glu Arg Asp Ile Arg Arg Tyr Ala Thr His Glu Arg Lys Met 340 345 350 Met Leu Asp Asn His Ala Leu Tyr Asp Lys Thr Lys Arg Val Phe Leu 355 360 365 Lys Ala Glu Ser Glu Glu Glu Ile Phe Ala His Leu Gly Leu Asp Tyr 370 375 380 Ile Glu Pro Trp Glu Arg Asn Ala 385 390 9 1148 DNA Calf thymus 9 atgagaggat cgcatcacca tcaccatcac aagactccac cacttgctgt aaaaaagatc 60 tcccagtacg cgtgtcaaag aaaaaccact ttgaacaact ataaccacat attcacggat 120 gcctttgaga tactggctga aaattctgag tttaaagaaa atgaagtctc ttatgtgaca 180 tttatgagag cagcttctgt acttaaatct ctgccattca caatcatcag tatgaaggat 240 acagaaggaa ttccctgcct gggggacaag gtgaagtgta tcatagagga aattattgaa 300 gatggagaaa gttctgaagt taaagctgtg ttaaatgatg aacgatatca gtccttcaaa 360 ctctttactt ctgtttttgg agtgggactg aagacatctg agaaatggtt caggatgggg 420 ttcagatctc tgagtaaaat aatgtcagac aaaaccctga aattcacaaa aatgcagaaa 480 gcaggatttc tctattatga agaccttgtc agctgcgtga ccagggccga agcagaggcg 540 gttggcgtgc tggttaaaga ggctgtgtgg gcatttctgc cggatgcctt tgtcaccatg 600 acaggaggat tccgcagggg taagaagatt gggcatgatg tagatttttt aattaccagc 660 ccaggatcag cagaggatga agagcaactt ttgcctaaag tgataaactt atgggaaaaa 720 aagggattac ttttatatta tgaccttgtg gagtcaacat ttgaaaagtt caagttgcca 780 agcaggcagg tggatacttt agatcatttt caaaaatgct ttctgatttt aaaattgcac 840 catcagagag tagacagtag caagtccaac cagcaggaag gaaagacctg gaaggccatc 900 cgtgtggacc tggttatgtg cccctacgag aaccgtgcct ttgccctgct aggctggact 960 ggctcccggc agtttgagag agacatccgg cgctatgcca cacacgagcg gaagatgatg 1020 ctggataacc acgctttata tgacaagacc aagagggtat ttctcaaagc ggaaagtgaa 1080 gaagaaatct ttgcacatct gggattggac tacattgaac catgggaaag aaatgcttaa 1140 gcttgcgc 1148 10 379 PRT Calf thymus 10 Met Arg Gly Ser His His His His His His Lys Thr Pro Pro Leu Ala 1 5 10 15 Val Lys Lys Ile Ser Gln Tyr Ala Cys Gln Arg Lys Thr Thr Leu Asn 20 25 30 Asn Tyr Asn His Ile Asp Ala Phe Glu Ile Leu Ala Glu Asn Ser Glu 35 40 45 Phe Lys Glu Asn Glu Val Ser Tyr Val Thr Phe Met Arg Ala Ala Ser 50 55 60 Val Leu Lys Ser Leu Pro Phe Thr Ile Ile Ser Met Lys Asp Thr Phe 65 70 75 80 Thr Glu Gly Ile Pro Cys Leu Gly Asp Lys Val Lys Cys Ile Ile Glu 85 90 95 Glu Ile Ile Glu Asp Gly Glu Ser Ser Glu Val Lys Ala Val Leu Asn 100 105 110 Asp Glu Arg Tyr Gln Ser Phe Lys Leu Ser Val Phe Gly Val Gly Leu 115 120 125 Lys Thr Ser Glu Lys Trp Phe Arg Met Gly Phe Thr Phe Arg Ser Leu 130 135 140 Ser Lys Ile Met Ser Asp Lys Thr Leu Lys Lys Met Gln Lys Ala Gly 145 150 155 160 Phe Leu Tyr Tyr Glu Asp Leu Val Ser Cys Val Thr Arg Ala Glu Ala 165 170 175 Glu Ala Val Gly Val Leu Val Lys Glu Ala Val Trp Ala Phe Leu Pro 180 185 190 Asp Ala Phe Val Thr Met Thr Gly Gly Phe Arg Arg Gly Lys Lys Ile 195 200 205 Gly His Asp Val Asp Phe Leu Ile Thr Ser Pro Gly Ser Ala Glu Asp 210 215 220 Glu Glu Gln Leu Leu Pro Lys Val Ile Asn Leu Trp Glu Lys Lys Gly 225 230 235 240 Leu Leu Leu Tyr Tyr Asp Leu Val Glu Ser Thr Phe Glu Lys Phe Lys 245 250 255 Phe Thr Leu Pro Ser Arg Gln Val Asp Thr Leu Asp His Phe Gln Lys 260 265 270 Cys Phe Leu Ile Leu Lys Leu His His Gln Arg Val Asp Ser Ser Lys 275 280 285 Ser Asn Gln Gln Glu Gly Lys Thr Trp Lys Ala Ile Arg Val Asp Leu 290 295 300 Val Met Cys Pro Tyr Glu Asn Arg Ala Phe Ala Leu Leu Gly Trp Thr 305 310 315 320 Gly Ser Arg Gln Phe Glu Arg Asp Ile Arg Arg Tyr Ala Thr His Glu 325 330 335 Arg Lys Met Met Leu Asp Asn His Ala Leu Tyr Asp Lys Thr Lys Arg 340 345 350 Val Phe Leu Lys Ala Glu Ser Glu Glu Glu Ile Phe Ala His Leu Gly 355 360 365 Leu Asp Tyr Ile Glu Pro Trp Glu Arg Asn Ala 370 375 11 1121 DNA Calf thymus 11 atgagaggat cgcatcacca tcaccatcac atctcccagt acgcgtgtca aagaaaaacc 60 actttgaaca actataacca catattcacg gatgcctttg agatactggc tgaaaattct 120 gagtttaaag aaaatgaagt ctcttatgtg acatttatga gagcagcttc tgtacttaaa 180 tctctgccat tcacaatcat cagtatgaag gatacagaag gaattccctg cctgggggac 240 aaggtgaagt gtatcataga ggaaattatt gaagatggag aaagttctga agttaaagct 300 gtgttaaatg atgaacgata tcagtccttc aaactcttta cttctgtttt tggagtggga 360 ctgaagacat ctgagaaatg gttcaggatg gggttcagat ctctgagtaa aataatgtca 420 gacaaaaccc tgaaattcac aaaaatgcag aaagcaggat ttctctatta tgaagacctt 480 gtcagctgcg tgaccagggc cgaagcagag gcggttggcg tgctggttaa agaggctgtg 540 tgggcatttc tgccggatgc ctttgtcacc atgacaggag gattccgcag gggtaagaag 600 attgggcatg atgtagattt tttaattacc agcccaggat cagcagagga tgaagagcaa 660 cttttgccta aagtgataaa cttatgggaa aaaaagggat tacttttata ttatgacctt 720 gtggagtcaa catttgaaaa gttcaagttg ccaagcaggc aggtggatac tttagatcat 780 tttcaaaaat gctttctgat tttaaaattg caccatcaga gagtagacag tagcaagtcc 840 aaccagcagg aaggaaagac ctggaaggcc atccgtgtgg acctggttat gtgcccctac 900 gagaaccgtg cctttgccct gctaggctgg actggctccc ggcagtttga gagagacatc 960 cggcgctatg ccacacacga gcggaagatg atgctggata accacgcttt atatgacaag 1020 accaagaggg tatttctcaa agcggaaagt gaagaagaaa tctttgcaca tctgggattg 1080 gactacattg aaccatggga aagaaatgct taagcttgcg c 1121 12 370 PRT Calf thymus 12 Met Arg Gly Ser His His His His His His Ile Ser Gln Tyr Ala Cys 1 5 10 15 Gln Arg Lys Thr Thr Leu Asn Asn Tyr Asn His Ile Asp Ala Phe Glu 20 25 30 Ile Leu Ala Glu Asn Ser Glu Phe Lys Glu Asn Glu Val Ser Tyr Val 35 40 45 Thr Phe Met Arg Ala Ala Ser Val Leu Lys Ser Leu Pro Phe Thr Ile 50 55 60 Ile Ser Met Lys Asp Thr Phe Thr Glu Gly Ile Pro Cys Leu Gly Asp 65 70 75 80 Lys Val Lys Cys Ile Ile Glu Glu Ile Ile Glu Asp Gly Glu Ser Ser 85 90 95 Glu Val Lys Ala Val Leu Asn Asp Glu Arg Tyr Gln Ser Phe Lys Leu 100 105 110 Ser Val Phe Gly Val Gly Leu Lys Thr Ser Glu Lys Trp Phe Arg Met 115 120 125 Gly Phe Thr Phe Arg Ser Leu Ser Lys Ile Met Ser Asp Lys Thr Leu 130 135 140 Lys Lys Met Gln Lys Ala Gly Phe Leu Tyr Tyr Glu Asp Leu Val Ser 145 150 155 160 Cys Val Thr Arg Ala Glu Ala Glu Ala Val Gly Val Leu Val Lys Glu 165 170 175 Ala Val Trp Ala Phe Leu Pro Asp Ala Phe Val Thr Met Thr Gly Gly 180 185 190 Phe Arg Arg Gly Lys Lys Ile Gly His Asp Val Asp Phe Leu Ile Thr 195 200 205 Ser Pro Gly Ser Ala Glu Asp Glu Glu Gln Leu Leu Pro Lys Val Ile 210 215 220 Asn Leu Trp Glu Lys Lys Gly Leu Leu Leu Tyr Tyr Asp Leu Val Glu 225 230 235 240 Ser Thr Phe Glu Lys Phe Lys Phe Thr Leu Pro Ser Arg Gln Val Asp 245 250 255 Thr Leu Asp His Phe Gln Lys Cys Phe Leu Ile Leu Lys Leu His His 260 265 270 Gln Arg Val Asp Ser Ser Lys Ser Asn Gln Gln Glu Gly Lys Thr Trp 275 280 285 Lys Ala Ile Arg Val Asp Leu Val Met Cys Pro Tyr Glu Asn Arg Ala 290 295 300 Phe Ala Leu Leu Gly Trp Thr Gly Ser Arg Gln Phe Glu Arg Asp Ile 305 310 315 320 Arg Arg Tyr Ala Thr His Glu Arg Lys Met Met Leu Asp Asn His Ala 325 330 335 Leu Tyr Asp Lys Thr Lys Arg Val Phe Leu Lys Ala Glu Ser Glu Glu 340 345 350 Glu Ile Phe Ala His Leu Gly Leu Asp Tyr Ile Glu Pro Trp Glu Arg 355 360 365 Asn Ala 370 

We claim:
 1. An enzyme comprising a truncated terminal deoxynucleotidyl transferase (TdT) derivative, wherein the derivative is N-terminally truncated by up to 161 amino acids in comparison to native TdT and has a 20- to 30-fold higher enzyme activity in solutions containing Co²⁺ ions.
 2. An enzyme as claimed in claim 1, wherein the derivative is shortened at the N-terminus by 100 to 160 amino acids compared to native TdT, has a molecular weight between 36 and 46 kDa′ (SDS page) and is obtainable from calf thymus.
 3. An enzyme as claimed in claim 1, wherein the derivative is shortened at the N-terminus by 138 amino acids, has a molecular weight between 44 and 46 kDa (SDS page) and has a 20- to 25-fold increased enzyme activity in solutions containing Co²⁺ ions compared to native TdT.
 4. A method for the recombinant production of a truncated TdT from calf thymus as claimed in claims 1 to 3 comprising the steps of: a) transformating a host cell with an expression plasmid containing a nucleic acid fragment which codes for an N-terminally truncated TdT fragment and is optionally fused with a nucleic acid which codes for a protein tag that facilitates the subsequent purification, b) culture of the host cell to express the recombinant truncated TdT under suitable culture conditions for the respective host cell, c) isolating the recombinant truncated TdT from the host cell and d) use and examination of the recombinant truncated TdT in a functional assay.
 5. A method as claimed in claim 4, wherein the expression plasmid contains a nucleic acid fragment according to SEQ ID NO.: 7, SEQ ID NO.: 9 or SEQ ID NO.:
 11. 6. A method as claimed in claim 4, wherein the nucleic acid fragment codes for a truncated terminal transferase according to SEQ ID NO.: 8, SEQ ID NO.: 10 or SEQ ID NO.:
 12. 7. A method as claimed in claim 4, wherein the expression plasmid contains a nucleic acid fragment according to SEQ ID NO.:
 7. 8. A method as claimed in claim 4, wherein the truncated terminal transferase contains the amino acid sequence according to SEQ ID NO.:
 8. 